Porous ceramic membranes are durable film materials having a variety of industrial and scientific uses, the most common of which is use in separation processes. Although organic membranes are currently used most often for industrial separation processes, metal oxide ceramic membranes offer several advantages over organic membranes. Metal oxide ceramic membranes have a greater chemical stability, since they are resistant to organic solvents, chlorine, and extremes of pH, to which organic membranes may be susceptible. Ceramic membranes are also inherently more stable at high temperatures, to allow efficient sterilization of process equipment not possible with organic membranes and to allow for operation at these elevated temperatures, e.g., above 200.degree. C., at which no organic membrane can function. Metal oxide ceramic membranes are also entirely inorganic, so they are generally quite stable and resistant to microbial or biological degradation which can occasionally be a problem with organic membranes.
The nature of the material results from the general procedure for making such membranes. Metal oxide ceramic membranes are formed through a process beginning with organic-inorganic molecules which are formed into small metal oxide particles, then fused into a unitary ceramic material. On a microscopic level, the materials may be conceptualized as a series of generally uniform spherical particles which are arranged in a close packing model, with the junction points between the spherical particles being fused together. The result is a durable inorganic, homogenous, amorphous to crystalline material which has a relatively uniform distribution of pores, with the pores being determined by the size of the particles forming the membrane. The gaps between the fused particles form a series of pores so that the membrane is porous. The smaller the size of the particles, the smaller the holes left between them, when the particles are packed together and fused.
The general approach to the manufacture of metal oxide ceramic membranes consists of a sol-gel process. In the sol part of the process, a dilute colloidal solution or suspension of metal oxide particles is created. The metal oxide is typically initiated into the process as a metal alkoxide dissolved in an alcohol solvent. The introduction of the metal alkoxide to water with rapid stirring results in the hydrolysis of the metal to metal hydroxide monomers, dimers, polymers, and/or particles, depending on the quantity of water used. Insoluble metal oxide particles are then peptized by the addition of an acid which causes the particles of the metal oxide to have a greater propensity to remain in suspension, presumably due to charges acquired by the particles during the peptizing process. This process is one of charge stabilization. Stabilization could also be accomplished sterically by adding surfactant agents. Care must be taken at this stage to prevent accretion of large particles, if a small pore size is desired in the membrane. Alternatively, an aqueous sol may be produced by hydrolyzing a metal alkoxide or a metal salt.
Then, under very tightly controlled conditions, the alcohol or aqueous solvent is removed from the colloidal sol, resulting in a semi-solid phase of material known as a xerogel or gel. The gel is typically a translucent or transparent semi-solid material which will retain its shape, but is still relatively deformable. Removal of the remaining water and solvent, and sintering of the gel results in a durable rigid ceramic material which can either be formed as an unsupported membrane or as a supported membrane coated onto a substrate, which, in turn, can be either porous or non-porous, and metallic or non-metallic, depending on the particular application.
One desirable metal element for use in such a metal oxide ceramic membrane is titanium. Titanium is attractive since it has catalytic and photocatalytic qualities that make a titanium oxide ceramic membrane useful for chemical or photoelectrochemical processes in which a less catalytic or photocatalytic metal oxide ceramic membrane would not be suitable. Also, titanium oxide ceramic membranes are typically transparent or lightly colored, thereby giving them desirable optical properties for certain applications in which transparency is an asset.
Practical limitations on the use of such metal oxide ceramic membranes have included the absolute size and the range of size of the pores which can be created in the metal oxide membranes. Clearly, if a membrane is to be used for filtration or other form of separation, the size and the variance in size of the pores through the membrane are a critical factor in the suitability of the membrane for the particular separation function desired. There must be limitations on the heat of the sintering process, since too high a temperature will destroy the pores, but, within a wide range, a porous ceramic material can be created as a supported or as an unsupported membrane.
At least one teaching is known, by the inventors here, of a method for preparing polymeric or particulate titanium ceramic membranes by a process which allows the reproducible and predictable fabrication of titanium ceramic membranes and which permits crack-free membranes of predictable qualities to be created. As disclosed in international published PCT patent application WO 89/00983, the method for creating particulate ceramic membranes involved the use of relatively large amounts of water and a mild heating during the peptizing step to create the appropriately charged particles which could then be dewatered and sintered to create a titanium oxide ceramic membrane.
The method for creating the polymeric ceramic membranes included strictly limiting the amount of water included in the reaction vessel so as to foster the creation of covalent bonds between the titanium and oxygen molecules in the suspension, and also required the use of an alkyl alcohol different from the alkyl alcohol in the titanium alkoxide for the process to be effective.
Certain attention has been directed toward the creation of porous ceramic membranes with exceedingly small pore size. An example of such research is disclosed in U.S. Pat. No. 5,006,248. Similar work is described in Anderson et al., Journal of Membrane Science, 39, pp. 243-458 (1988). The process described in the above patent enables the creation of porous ceramic membranes with small pore sizes, either as supported or unsupported materials.
Metal oxide ceramic membranes of transition metals can also be used for catalytic purposes. U.S. Pat. No. 5,035,784 describes how such materials can be used under ultraviolet light to degrade polychlorinated organic chemicals. Doping can be utilized in mixed membrane materials to increase electrical conductivity for various catalytic purposes. U.S. Pat. No. 5,028,568 describes the doping of titanium membranes with niobium to achieve increased electrical conductivity.
Practical utility of ceramic membranes requires large, thin, crack-free surfaces which can be difficult to reliably make in the unsupported form, due to the frailty of the ceramic material. Therefore, supported membranes are more practical for most applications. Traditionally, the accretion or layering of such very small size ceramic particles onto a porous substrate has turned out not to be a trivial endeavor. Such particles tend to accrete, or deposit, on a substrate in an irregular manner resulting in nonhomogeneous thickness. The pores of the substrate which the microporous membrane must span are much larger than the colloidal particles which make up the membrane itself. In addition, the surface topography and electrochemical character of the substrate can adversely affect the deposition of the particles in the accumulating membrane on the substrate. Since the object of depositing such a membrane on a porous substrate is to create a material which can be used for filtering, a highly uniform size distribution of pores in the resulting porous ceramic membrane and a thin, uniform thickness of the membrane are desired.